This invention relates to circuits for and methods of efficiently operating lamps which utilize electromagnetic field inducing means to ionize a gas within a sealed envelope in order thereby to activate fluorescent material and produce light. Much research has been performed in the field of electrodeless light sources inasmuch as they require less electrical power to operate than conventional incandescent lamps and are generally several times more efficient than incandescent lamps on a lumens per watt basis. Electrodeless lamps usually comprise an input winding surrounding a toroid or annular core, (the combined coil and core serving as the inducing means when current flows such that an electromagnetic field is produced), and an ionizable gaseous radiation medium within a non-oxidizing atmosphere contained within a lamp envelope. The gas is ionized by the inducing means which results in radiation from the ionized gas exciting lamp phosphors provided on the inner surface of the envelope or alternatively on the outer surface of the core, which in turn causes emission of visible light. The ionized gas is usually not relied upon to produce substantial visible light emission, but rather to produce radiation which causes visible light to be emitted from the thereby excited fluorescent phosphor. As is well known, this approach is significantly more energy efficient than incandescent lighting. Electrodeless lamps usually utilize a core of iron or ferromagnetic material. Examples of such cores are disclosed in U.S. Pat. No. 1,534,251. However, as brought out in U.S. Pat. No. 4,005,330, air core and iron core transformers are subject to relatively large heat losses. Thus, they are not practical for operation at radio frequencies ordinarily required for efficient operation of gaseous discharge lamps; and ferrite or similar core materials are better suited to provide the high permeability and low internal heat loss desired at radio frequency operation of such lamps. The term "radio frequencies" as used herein is intended to mean all frequencies above the humanly audible range up to approximately 10 MHz unless otherwise stated. As is well known, a ferrite is a ceramic-like material possessing ferromagnetic properties. Ferrites, as with all ferromagnetic core materials, are subject to undesirable energy losses in the form of heat when magnetic saturation is approached in operation. Additionally, in the above-noted U.S. Pat. No. 4,005,330, 50 kHz is disclosed as an appropriate operation frequency for electric field inducing means including ferrite cores.
The typical B-H magnetization curve for a magnetic core of the type used in electric field inducing means of electrodeless lamps is well known in the art. In the operation of prior electrodeless lamps including electric field inducing means comprising ferrite cores and windings, the cores were permitted to operate in the saturation region of the first quadrant of the typical B-H magnetization curve. This mode of operation results in substantial heat losses and thus inefficient energy utilization. Additionally, when the core becomes saturated, the inductance becomes substantially zero and the current accordingly rises rapidly. For example, a peak current of 3 amperes might occur at saturation, while the peak current might be one ampere when the core is not at saturation. Thus, by avoiding saturation, rapid current rises can be minimized or avoided, desirably enabling the use of less costly lower current rated components.
One prior approach for avoiding core saturation as disclosed in U.S. Pat. No. 4,002,999 involves using a detector to detect on-coming saturation and produce a reverse-bias input current through the core to prevent full core saturation. However, this approach requires use of a relatively complex detection circuit.
In normal operation of electrodeless lamps, a reverse EMF or flyback voltage is induced in the winding of the electric field inducing means as a result of the normal periodic "on/off" operation of a controlling means. This results in a momentary voltage across the winding which is opposite in polarity to, and of a magnitude several times larger than, that which appears across the winding just before the switch is opened. This voltage has been utilized to produce high voltage firing of the lamps and to keep them lit, but generally a substantial portion of energy is wastefully dissipated in the circuit externally of the lamp winding.